This invention relates generally to pressure-tight vessels for handling large volumes of a product as the product is treated through a thermal cycle with an upper temperature in excess of approximately 700 degrees Fahrenheit. In particular, the invention relates to pressure-tight vessels that could be used for delayed petroleum coking.
Delayed petroleum coking is a process in which a petroleum fraction is heated to a temperature at which it thermally decomposes to provide a solid coke product and a hydrocarbon distillate product. In general, a liquid petroleum feed stock is first distilled until the lighter ends have been recovered and a heavy residuum remains. This heavy residuum is generally preheated to a temperature of at least about 700.degree. F. before being fed to pressure-tight vessel. In the vessel, it may be further heated to temperatures up to 1000.degree. F. under high-pressure conditions that prevent the petroleum fraction from vaporizing until it has partially decomposed.
The decomposition process produces hydrocarbon vapors vessel and a heavy tar that continues to decompose until a porous coke is left in the vessel. The vaporization results in pores and channels in the residuum that may be filled with additional residuum.
Once the vessel is filled, the residuum is allowed to cool, forming coke. The coke may then be purged with steam to remove any remaining volatile components.
To complete the process, water is added to the vessel to quench the coke. As the level of the water progressively rises in the vessel, it quenches the coke to a temperature below 200.degree. F.
In order to increase production speed, the quenching operation is often done as quickly as possible. Unfortunately, the faster the coke is quenched, the greater the wear and tear on the vessel. One of the primary causes of this wear and tear is that the steel plate and the weld material joining the rings of plate that form the vessel have different yield and creep strengths. At the circumferential weld sites, thermal cycling causes progressively increasing permanent strains, eventually leading to distortion and cracking in or near the welds and often bringing to an end the useful life of the vessel.
As described in U.S. Pat. No. 3,936,358, some efforts have been made to reduce the wear and tear on vessels by controlling the quench rate. Unfortunately, this can reduce production speed.
More recently, efforts has been made to adjust the composition of the weld material so that its yield strength more closely matches that of the adjacent steel plates. It is hoped that a closer match will reduce the stresses at the weld sites, extending the life of the vessel. Unfortunately, vessels using the new weld materials have not been in service long enough to know whether this solution will be successful.
There remains a need for a pressure-tight vessel that can better withstand extreme thermal cycling.